On 19 August 2016, an intensive observation period (IOP) was performed to capture a detailed evolution of the vertical structure of the lower troposphere on and around Mount Washington (1917 m asl). In addition to the regular meteorological observations that are taken at the summit (temperature, relative humidity, wind, pressure, sky coverage, cloud type, etc.) and several mesonet sites on the east and west slopes (temperature, relative humidity, and wind), radiosondes were launched from the base of the Auto Road from pre-sunrise to post-sunset. Concurrently, water vapor stable isotopes were measured by a water vapor isotope analyzer that was driven in a pick-up truck up and down the Auto Road.
During the early morning of the IOP, a 15-20 m s-1wind forced residual layer air up the slopes to the summit from an altitude as low as 1400 m asl. Meanwhile, radiosonde temperature and dewpoint profiles indicate the boundary layer top was below the summit elevation in the free atmosphere. Winds diminished by 1000 Eastern Standard Time (EST), and highly variable summit water vapor isotope ratios and dewpoint values indicate the entrainment zone descended upon the summit. Around 1200 EST, the convective boundary layer grew through the summit elevation as suggested by a rapid increase in dewpoint, base-to-summit lapse rates at or exceeding dry adiabatic, and homogenizing of wind direction at sites above treeline. This IOP reveals important thermal and air mass height differences between the free atmosphere and along mountain slopes and provides guidance on methods to observe these differences. This research suggests that most mountain ranges around the world have an elevational range that experiences regular variability in exposure to the boundary layer, entrainment zone and free troposphere that help to characterize local temporal changes in climate and air quality.